This invention relates to apparatus and methods suitable for stripping volatile organic compounds (VOC's) from solid materials, e.g. detoxifying soils contaminated with volatile hydrocarbons or other volatile organic compounds.
Soils become contaminated by hydrocarbons and other organic compounds in many ways, including leakage from underground storage tanks or spillage during trucking and shipping. Contaminated soils should be treated, e.g., to prevent contamination of water supplies or nearby plants and animals, including humans. One treatment involves extremely high temperatures (2000.degree. F.) and requires substantial energy consumption to volatilize and breakdown the organic compounds to form relatively non-toxic gases. Alternatively, volatilized organic compounds are adsorped to solid material. In the former case, the volatile organic compounds are destroyed by prolonged heating at extremely high-temperatures so as to completely oxidize them to CO.sub.2, H.sub.2 O and other relatively non-toxic substances. In the latter case, the solid material must be disposed of.
For example, Hazardous Waste Consultant (1988, p. 1-16 to 1-20) describes new technology for removing volatile organic compounds ("VOCs") from soils. One process involves shredding and aerating the contaminated soil. The other involves use of a heat exchanger to dry and heat the soil to about 450.degree. F. "At this temperature, the VOCs (volatile organic compounds) are vaporized and can either be destroyed through subsequent high-temperature incineration or recovered via condensation and adsorption on activated carbon." In the described system, the soil is heated by two "hot screws" which have hot oil circulating through them. Energy to heat the soil is provided by a 6-million-BTU/hour hot oil heater. "Combustion of propane and VOCs from the system heat the circulating oil to 640.degree. F." VOCs from the treated soil are passed through a baghouse filter, a condenser and then an after burner (fired by propane) for thermal treatment. Condensates from this treatment are either removed directly as oils, or adsorbed to carbon. See, e.g. U.S. Pat. No. 4,738,206.
Eatherton (U.S. Pat. No. 4,431,405) describes use of a rotary dryer for depositing hydrocarbon contaminants onto wood. The dryer is heated with hot air of 1,100.degree. F.
The energy demand imposed for destroying volatile organic compounds is a significant drawback to processes with that requirement.